1. Field of the Invention
The present invention relates to a linear actuator responsive to a pressure fluid introduced from fluid outlet/inlet ports for reciprocally moving a slide table in an axial direction of an actuator body.
2. Description of the Related Art
Linear actuators have heretofore been used as means for conveying workpieces or the like. Linear actuators are capable of linearly reciprocally moving a movable table along a cylinder body for thereby conveying a workpiece placed on the movable table.
One known linear actuator is disclosed in Japanese laid-open utility model publication No. 5-42716, for example. FIG. 19 of the accompanying drawings illustrates the disclosed linear actuator in the form of a fluid pressure cylinder assembly. As shown in FIG. 19, the fluid pressure cylinder assembly comprises a cylinder body 1, a pair of cylinder chambers 2, 3 defined in the cylinder body 1 in a horizontal juxtaposed array, an oblong hole 4 defined in the cylinder body 1 between the cylinder chambers 2, 3 in communication therewith and opening at an upper surface of the cylinder body 1, the oblong hole 4 having a longitudinal axis parallel to the horizontal juxtaposed array of cylinder chambers 2, 3, a pair of pressure supply ports (not shown) communicating with the cylinder chambers 2, 3, respectively, a pair of pistons 5, 6 reciprocally movably inserted in the respective cylinder chambers 2, 3 for simultaneous movement in the same direction, a rod 7 sandwiched between and held in linear, but not bonded, contact with the pistons 5, 6 and having a vertical central axis, for movement in the oblong hole 4 in unison with the pistons 5, 6, a table 18 placed over the cylinder body 1 and connected to an upper end of the rod 7, a circulatory ball bearing (not shown) disposed in the table 18, and a track groove (not shown) defined in the cylinder body 1 and engaging the circulatory ball bearing. The linear actuator also has a pair of end covers 8, 9 mounted respectively in the opposite horizontal ends of the cylinder body 1 and defining respective pressure chambers 14, 15 in the cylinder chambers 2, 3 together with the pistons 5, 6, respectively.
In the conventional fluid pressure cylinder assembly shown in FIG. 19, the cylinder chambers 2, 3 in which the respective pistons 5, 6 are reciprocally movable are formed by a through hole defined in the cylinder body 1. The cylinder body 1, which serves as a guide member for the circulatory ball bearing, is required to be made of a highly hard material that is highly resistant to wear due to frictional contact with the circulatory ball bearing. Generally, the high hardness requirements imposed on the cylinder body 1 are met by making the cylinder body 1 of steel and heat-treating the cylinder body 1. Since, however, the cylinder chambers 2, 3 are deformed by the heat treatment, the cylinder body 1 needs to be machined to removes the strains of the cylinder chambers 2, 3 after it has been heat-treated. As a result, the process of manufacturing the conventional fluid pressure cylinder assembly has been complex, and hence the cost of the conventional fluid pressure cylinder assembly is high.
In the conventional fluid pressure cylinder assembly, reciprocating movement of the pistons 5, 6 is transmitted to the table 18 through the rod 7 which is sandwiched between and held in linear, but not bonded, contact with the pistons 5, 6. Therefore, when a large load is posed on the table 18 due to the weight of a workpiece placed thereon, the load is transmitted to the joined regions between the pistons 5, 6 and the rod 7 which extend perpendicularly to each other. Inasmuch as the rod 7 is sandwiched between and held in linear, but not bonded, contact with the pistons 5, 6, the contact surfaces of the pistons 5, 6 and the rod 7 are worn relatively rapidly in usage over a long period of time, resulting in a reduction in the accuracy with which the table 18 moves.